HTTPkit adapter

This is pretty much WIP, since I'm about to give #168 another push. It also lacks response streaming.

Hatetepe will soon be superceded by [HTTPkit](https://github.com/lgierth/httpkit), which is close to its first release. It offers largely the same API and functionality, but is actually a complete rewrite with a focus on extensibility and high code quality.

I should probably also add a deprecation notice for Hatetepe.